The present invention relates to a carbon nanotube and to an apparatus provided therewith.
A carbon nanotube is a microscopic material composed mainly of carbon. It has a very large aspect ratio, with its diameter being a few nanometers and its length being hundreds to thousands of nanometers. Therefore, it is expected to be applied to scanning probe microscopes (SPM), typified by atomic force microscopes (AFM) and scanning tunneling microscopes (STM), which depend for their resolution largely on the radius of curvature of the tip of the sensing probe.
Among SPMs, there is a magnetic force microscope (MFM) which employs a ferromagnetic sensing probe to read a magnetic gradient in a sample. When a carbon nanotube, which is a paramagnetic substance having the stable σ-π bond, is to be applied to an MFM, it needs to have ferromagnetism. This objective can be achieved by attaching, in any way, a ferromagnetic metal to the tip or inside of the carbon nanotube. The foregoing technique is applicable not only to an MFM, but also to any apparatus requiring ferromagnetism.
A method of imparting ferromagnetism to the tip of a carbon nanotube is disclosed in Japanese Patent Laid-open No. 321292/2000, and a method of causing a carbon nanotube to include a ferromagnetic metal is disclosed in Japanese Patent Laid-open No. 89116/2001.
Unfortunately, a composite material, which is composed of a carbon nanotube and a ferromagnetic metal, has a problem concerning the strength of the bond between them. If a carbon nanotube with iron as a ferromagnetic metal is used as a sensing probe for an MFM, there is the possibility that iron particles will drop off, or that the probe will be broken, when the probe is brought into contact with a sample. Such a problem would occur when the shape of the sample is measured by use of the tapping mode, in which the sample is tapped with a vibrating probe, or by use of the contact mode, in which the sample is scanned with a probe in contact with it.
There is another problem which relates to the temperature at which the probe is used. For example, a probe with iron as a ferromagnetic metal has difficulties in the measurement of magnetism at high temperatures exceeding 770° C., which is a Curie point of iron. It is difficult to carry out stable measurement at high temperatures, even under the Curie point, because the amount of magnetization usually decreases with temperature, thereby resulting in noise. The foregoing problem also arises when the probe comes into contact with a sample, thereby generating heat.